iPods and similar portable jukeboxes are creating a generation of kids with “old ears,” according to CNN.

To document the trend, [Purdue researcher Robert Novak] and colleagues have been randomly examining students and found a disturbing and growing incidence of what is known as noise-induced hearing loss. Usually, it means they’ve lost the ability to hear higher frequencies, evidenced at times by mild ear-ringing or trouble following conversations in noisy situations.

The phenomenon is most pronounced with people in their 30s and 40s, who used Walkmen when they first came out. Apparently, it’s not just the volume level, either — it’s also the length of time you listen. Keep it at an hour a day, at 60% of max volume, at most, if you don’t want to end up wearing a hearing aid to your 30th birthday party.